Solar cells include a monocrystalline silicon solar cell, a polycrystalline silicon solar cell, an amorphous silicon solar cell (these are silicon solar cells), a GaAs solar cell, a CdS/CdTe solar cell, a CIS solar cell (these are compound solar cells), a dye-sensitized solar cell (organic solar cell), etc.
Of those, a CIS solar cell is polycrystalline and is therefore favorable for being upsized or mass-produced, and is characterized in that its forbidden band width can be freely varied depending on the material thereof and that it has a theoretical conversion efficiency (31%) on the same level as the theoretical conversion efficiency of a silicon solar cell and has a lifetime (about 20 years) on the same level as that of a silicon solar cell. Accordingly, this is specifically noted as a potential solar cell, and also in Japan, mass-production of CIS solar cells has been started.
The CIS solar cell is a thin-film polycrystalline solar cell in which a group compound comprising Cu, In, Ga, Ag, Se, S and the like and referred to as a chalcopyrite compound is used as a material of the light absorbing layer thereof, and typically includes those comprising Cu(In,Ga)Se2 [GIGS], Cu(In,Ga)(Se,S)2 [CIGSS], CuInS2 [CIS] or the like.
FIG. 1 is a perspective view showing a general structure of a CIS solar cell 11. The CIS solar cell 11 comprises a lower electrode 13 of Mo provided on a substrate 12 of glass (soda lime glass), a light absorbing layer 14 of GIGS or the like formed on the lower electrode 13, and a transparent upper electrode 16 of ZnO or the like provided thereon via a buffer layer 15 of CdS or the like. Concretely, after the substrate 12 is washed, Mo or the like is sputtered on the upper surface of the substrate 12 to form the lower electrode 13 thereon, and further a chalcopyrite compound such as GIGS is deposited on the lower electrode 13 through simultaneous vapor deposition to form the light absorbing layer 14 thereon. Next, the substrate is dipped in a CdS solution to thereby grow the buffer layer 15 on the upper surface of the light absorbing layer 14, and ZnO or the like is sputtered onto the buffer layer 15 to form the upper electrode 16 thereon.
On the other hand, as illustrated in FIG. 2, it is well-known a silicon solar cell 21 which comprises a lower electrode 23 of ZnO formed on an electroformed Ni substrate 22 having, on its upper surface, unevenness 27 having a mean roughness of from 0.1 to 10 μm, a light absorbing layer of amorphous silicon formed on the lower electrode 23, and a transparent upper electrode 26 of ZnO provided thereon. This type of conventional art is disclosed in Japanese Patent Publication No. 2001-345460A (Patent Document 1).
In the silicon solar cell 21 of this type, the surface of the lower electrode 23 also has unevenness 28 owing to the unevenness 27 formed on the surface of the electroformed substrate 22. Accordingly, light 29 incident on the light absorbing layer 24 through the upper electrode 26 is scattered by the unevenness 28 and is then trapped and absorbed by the light absorbing layer 24, and the conversion efficiency of the silicon solar cell 21 is thereby increased. As the electroformed substrate 22 is used, unevenness can be formed on the surface of the substrate with low cost.    Patent Document 1: Japanese Patent Publication No. 2001-345460A